But Lobsang appeared at his side, breathing hard, his robe ripped at the neck. ‘It’s all right, Joshua. I’m under no real threat. This is just—’

‘HYY-AAGH!’ The guy on the ground did a kind of back flip, and once more launched himself through the air at Lobsang. But Lobsang dipped into his own forward roll, and the newcomer was sent flying. This time it was the assailant who stepped away, before he hit the ground.

Lobsang straightened up, breathing hard. ‘It’s one of Agnes’s ideas. You see—’

‘NYA-HAAH!’ Now the assailant, Cho-je, came back into the world above Lobsang’s head, with his fists clamped together and ready to slam down on Lobsang’s crown. But Lobsang ducked, whirled, and caught him with a kick in the stomach – and again Cho-je disappeared.

Joshua gave up. He holstered his weapon, stood back, and watched the fight. It was a blur of kicks, punches, even head-butts that rained in with hard, meat-slapping impacts, and of stepping, as the two figures popped in and out of existence, each trying to get the down on the other. During his travels with Lobsang Joshua had watched plenty of Jackie Chan movies. And out in the Long Earth he had been involved in his own battles with elves, stepping humanoids honed by the hunt, who could cross between the worlds with such precision that they could materialize alongside you with their hands already in position to close around your throat. This was something like all of that, he thought, hastily mashed together, a high-speed blur of action that was all but impossible to follow.

‘HEE-ARR-AARGH!’

‘Cho-je, you fool!—’

It ended when Lobsang grabbed Cho-je’s left hand, as if to shake it, and, holding on hard, executed a standing somersault. When it was done he was still holding the hand, which had been ripped off at the wrist. Cho-je, bemused, breathing hard, looked at the stump of his arm; Joshua saw LEDs spark amid a whitish fluid that dripped to the ground.

Cho-je bowed to Lobsang. ‘Nice work! Good to see Sister Agnes’s care has not softened you up!’

‘On the contrary,’ Lobsang said. ‘Until we meet again.’

‘Until then. If I may have my detached extremity . . .’ Lobsang gave him back the severed hand, and Cho-je snapped out of existence.

‘So, Lobsang – Cho-je?’

Lobsang was sweating, quite convincingly. ‘As I said, Agnes’s idea. She has the notion that I’m too powerful. I need challenges, she says. So I endure an endless routine of toughening up and training. Actually, Joshua, Agnes got the idea for Cho-je from my account of our sparring matches during our voyage on the Mark Twain. I do derive enormous benefit in terms of ambulant body control from such exercises, and Cho-je is an increasingly ingenious opponent. By the way, in addition to this training partner, she also recruited another, one of the past inmates from the Home, a rather reclusive young man who has devoted his life to launching ingenious computer-virus attacks on me.’

‘Viruses, huh?’

They began to walk back to the twain. ‘Viruses are a worse threat to me than any physical violence, no matter how many backups I create. Any synching between my iterations at all leaves me open to a potentially lethal attack. I’m thinking of installing at least one entirely non-electronic backup.’

‘Such as?’

‘Oh, a few hundred monks in a scriptorium somewhere, endlessly copying my thoughts from one bound paper volume to another. A scriptorium on the moon maybe.’

‘One thing has definitely changed about you, Lobsang. Your jokes are no better. But at least now I can tell they are jokes.’

‘I’ll take that as a compliment.’

‘And to think that just as this incident with Cho-je occurred you were about to lecture me on “common sense”.’

‘We can continue that discussion in the morning. The twain is relatively spartan but quite comfortable, I think you’ll find.’

‘Any good movies?’

‘Of course. Your choice. But nothing with singing nuns in, if you don’t mind . . .’

9

IN THE MORNING they ate breakfast in near silence, and flew on. Rather than make straight for the caldera Lobsang at first skirted it to the west, following the line of what remained of a south-to-north highway. As they edged closer to the caldera, the increasing thickness of ash began to overwhelm the landscape as it had existed before the eruption. They were entering a true volcanic province, Joshua thought, like a fragment of an alien world brought down to the Earth.

‘The civilization of Datum Earth will never recover,’ Lobsang murmured, as they peered down at the strange landscape.

Joshua grunted. ‘That seems a tough conclusion to come to. It’s only been a few years . . .’

‘But think about it. We’d already used up all the easily accessible ore, the oil, much of the coal. And the world was already suffering tremendous climate disruption because of all the industrial gases we spewed into the air. When Yellowstone’s effects finally fade, the best guess for the future is widespread instability, as the world seeks a new equilibrium after two massive environmental shocks, one human-induced, one volcanic.’

‘Hmm. So is this why there’s talk of rewilding?’

The idea was, when the winter finally receded from the Datum Earth, why not take the chance to heal the world? All the species that had been driven to extinction on the Datum still prospered in the neighbouring worlds (though once again, Joshua knew, on some of the Low Earths many of those creatures were already in trouble). So, in North America, you could bring back the mammoth and the wild horse and the bison and the musk ox, and the seals in the rivers and the whales in the oceans – just step specimens over, as infants perhaps, to the Datum. Similarly you could let the landscapes and seas recover to their natural state.

‘It’s a romantic idea,’ Lobsang said. ‘Of course there’s a great deal of work to do before the Datum is even safe.’

‘Such as, decommissioning nuclear power plants?’

‘And waiting for dams to fail, for drained wetlands to flood . . . It will take decades, centuries, for pollutants like heavy metals and radioactive waste to be reduced to safe levels. Even then, where we have driven roads or dug mines into the bedrock, the mark of mankind will linger for millions of years.’

‘Makes you proud.’

‘If you say so, Joshua. However, an effort to heal this world using the riches of its stepwise siblings seems a noble ambition, whatever the limitations in practice.’

At last, to the north of Yellowstone itself, they paused over what had once been a township. Little remained but a few scattered traces of foundations, the hint of a grid of streets protruding from the ash; much of the rest was buried completely.

Joshua checked a map display; in cheerful white, green and yellow, with finely drawn state and county lines, it displayed the vanished human landscape as it had once been. ‘This is Bozeman.’

‘Yes. Or was. I thought you’d like to see this, Joshua. I saw from the records that you and Sally went in here on the final day of the eruption itself, when the caldera collapsed. Stepping into danger, seeking to save lives at the risk of your own evanescent existence.’

‘We weren’t the only ones,’ Joshua said without emotion.

The twain dipped in the air, skimming over ground choked by an unknowable thickness of ash and pumice.

‘We are still perhaps fifty miles from the caldera,’ Lobsang said. ‘But this place like many others was caught by the final pyroclastic flow. The eruption ceased when the caldera chamber was empty of magma. The tower of smoke and ash in the air over the volcano abruptly collapsed, and superhot rock fragments came washing out across the landscape at the speed of sound, burying everything for tens of miles around.’

Joshua had been there; he remembered.